2. Cell growth Flashcards
What are the limits to cell growth?
- transport - diffusion (more trouble removing waste / absorbing nutrients - diffusion slow and inefficient over large areas)
- communication (communication from nucleus to organelles must be fast - large cell - large distance - slow)
- mRNA synthesis (larger cell - more DNA - would take too long to construct needed proteins to support large cell)
What do cells use to make bigger cells?
- vacuole / adipocites - not a part of metabolism but take up space (ex: plant cells)
- syncytia - many nuclei in one cell (fusion of multiple cells) (ex: skeletal muscle)
- polytene chromosomes - parallel copies of DNA stuck together - more than one mRNA possible for synthesis at the same time (ex: fruitfly salivary glands)
- helper cells - part of the cell but not active itself - act as support (ex: granulosa cells in ovarian follicle - produce hormones)
Explain what are polytene chromosomes
Polytene chromosomes - parallel **copies of DNA stuck **together - more than one mRNA possible for synthesis at the same time - high level of function
Do cells enlarge when organisms grow?
No, cells remain small but their numbers increases in numerous cell divisions
Explain the cell cycle
I. Interphase (G1, G2, G3)
II. Mitotic phase (P, M, A, T)
How does the cell cycle progress in early embryo cells?
Embryos decouple growth and cell divisions - cells divide but they get smaller to occupy the same space - CLEAVAGE - because of limiting resources at that stage
What molecules are used to control the cell cycle?
Cyclins - allow the cell to progress to the next stage
Explain how do cyclins work
- cyclins work by controlling cyclin-dependent kinases (cdks) - enzymes which activate (by phosphorylation) progression to next stage of cell cycle - cyclins bind to cdks
- there are distinct restriction points (checkpoints) in the cycle - held at those points until required
What are the key requirements for the cell to progress to the next phase in the cell cycle?
- enough resources (ATP, nucleotides, am. a. for synthesis)
- no signal ‘don’t progress’ - inhibitors
- signal ‘do progress’ - growth factors
What is anisotropic growth?
Anisotropic growth - differential growth - when growth rates are not equal in all directions
Is environment important for control of cell growth? How does it influence growth?
- environmental control can directly drive morphogenesis
- nutrient availability (can determine colony shape), agar stiffness (how easy can spread)
What is directional cell division?
Directional cell division determines the tissue structure as cells divide in a specific direction to form a needed tissue structure - important for multicellular organisms (especially for non moving cells)
What are the possible types of division based on the division plane?
- periclinal (inreases radius)
- anticlinal (increases circumference)
- transverse (increases length)
How is an extra layer of tissue formed in mutant plant due to division plane change?
Periclinal divisions -> two cell types switch to anticlinal, one remains ant periclinal -> extra layer
Explain Hertwig’s rule
Hertwig’s rule: cells orientate their division planes in the direction that will reduce the existing mechanical stress in tissues (ex: heavy earings -> earlobes stretch by directional cell division to relieve the mechanical stress; pregnant people -> skin growth to relieve the stretch)
Does cell crowding influence cell assembly?
Yes, crowding of cells orientates expansion: uncrowded - random; crowded - orientated domains
Example: how does mesentry influence gut folding?
- mesentry - sheet of cells - connects gut to the body
- when mesentry was removed in chicken embryo - gut didn’t fold properly
- mechanism: mesentry doesn’t expand - only gut - folding occurs because from two connected tissues only one expands (rubber band experiment)
Explain planar cell polarity vs apico-basal polarity
- planar polarity: x-y plane
- apico-basal polarity: z axis
Explain what are astral microtubules
Astral microtubules - in metaphase microtubules assemble metaphase plate - astral microtubules outside the organising centres towards cell membrane
What is the function of astral microtubules?
To orientate the poles of cell division (planar cell polarity) - acts in trapping - attach to localised cortical microtubule binding proteins
How do cells know which polarity to choose for oriented mitosis?
Cell polarity is communicated between adjacent cells - astral microtubules pulled towards specific protein complexes
Give an example of orientated mitosis
- in neural tube closure - oriented mitosis to fuse the edges of the fold
- don’t fuse if insufficient folic acid - spina bifida
Why are people generally getting taller over centuries?
Improved healthcare and nutrition - affects overall cell division
Explain feotal transfusion syndrome, why it happens?
Monozygotic twins share the placenta - one might have more cords - get disproportionately more nutrients - twins of diff. development / size
What has the biggest influence on height / size of animal?
Genetics - can vary within same species
What is sexual dimorphism
- within same species sexes show differences in ex: size
- usually **females>males **because have to produce eggs / bear offspring but also males>females when men in competition
What does the vitruvian man depict?
Vitruvian man depicts that the human body has specific proportions that are more or less followed - growth guidance
What controls organism’s growth to its full size?
Growth hormone
What happens when the growth hormone disfunctions?
An organism grows into a **functional body **but not full size - Laron syndrome
What are the two main causes of growth hormone disfunction?
- no growth hormone / insufficient
- no growth hormone receptor
What organ produces growth hormone in mammals?
Pituitary gland
Explain how does the body control equal limb length
In limbs - growth plates - when bones grow the distance between cells and growth plate increases - distance makes signalling less efficient - cells stop dividing <= feedback loop
Explain rabit leg experiment used to investigate how equal limbs are grown
- growth of one leg of young rabit inhibited by cutting off circulation
- the other leg grows normally but the inhibited leg doesn’t - not enough resources
- circulation released - inhibited leg catches up
=> growth plates signal by distance for cells to keep dividing
What is achondroplasia?
Achondroplasia - most common form of skeletal dysplasia - impairs the growth of bone in the limbs - does not affect the trunk - skin grows as much as bones
Give examples that applying mechanical force to human skin drives skin growth
- pregnancy
- obesity
- cultural practices
How can the angle at which the force is applied afect the deformation / tension division?
Physics: corner / angle at which the force is applied affects the tension spread -> ex: square plane windows would crack - round windows more evenly spread tension, so don’t crack
What pathway is responsible for cell division under mechanical stress?
- Piezo1 - mechanosensitive ion channel protein - responds to stretch activated currents - drives cell division in needed direction of mechanical stress
How is it determined if cells should divide or die to accomodate mechanical stress?
Regulation of cell death/division occurs corresponding to a ratio of cells in a particular place - if too crowded cell death - if too sparse cells divide
Why organs don’t experience mechanical stretch when organisms move?
Organs are mechanically isolated - no stretch
Explain spleen experiment regarding regulation of the amount of organ tissue in an organism
Spleen tissue regulates its amount in an organism by cell communication => total mass of spleen = one normal spleen
What organ exhibits the opposite regulation of amount of tissue in an organism than spleen?
Thymus - when two feotal thymus grafted - two separate normal size thymus formed -> cell don’t communicate how much thymus tissue exists in an organism
Explain what is quorum sensing
Quorum sensing - type of cell-cell communication which allows to share info about cell density and adjust gene expression to it (autocrine signalling)
Explain how quorum sensing operates, give example
Cell-cell communication is achieved by certain thresholds of needed signalling molecules (the more cells - the more signalling molecules - the stronger signal - response only when threshold reached -> needed signal induced (ex: Wnt4 expression in kidney cells to differentiate)
Experiment for evidence for quorum sensing
- Wnt4-/- knockout
- normal levels of Wnt4
- Wnt4 signal faked by Li+ ions -> small aggregates formed needed structures -> small structures as the signalling induced not when enough cells were present
Explain trophic theory in cell growth
Trophic theory : cells need signals from other cells to inhibit cell death (survival factors secreted by other cells - detected - elective cell death stopped)
Trophic theory: explain the experiment of chick limb innervation
- Trophic theory - cells need signals from other cells to inhibit cell death
- neurons grow into limbs - when limb buds cut off - little motor neurons (cells need signals (neurotrophin) from other cells to inhibit death)
- when two limb buds grafted in - extra motor neurons formed - the signal (neurotrophin) intensity for neurons increased with extra muscle tissue => proves trophic theory (when less adjacent diff tissues - less tissue in same location formed)
Explain how the number of motor neurons is regulated in response to neurotrophin (quorum sensing)
1) Mitosis occurs despite presence / lack of neurotrophin - neurotrophin independent - fixed max point of cells reached
2) Depending on levels of neurotrophin cell death is regulated - neurotrophin dependent
